1. Field of the Invention
The present invention relates to a rare-earth alloy based binderless magnet and a method for producing such a magnet. More particularly, the present invention relates to a magnet produced by compacting a powder of a rapidly solidified rare-earth magnetic alloy under an ultrahigh pressure.
2. Description of the Related Art
Bonded magnets, obtained by adding a resin binder to a magnetic powder of a rapidly solidified rare-earth alloy, achieve high size precision and show great flexibility in shape, and have been used extensively in various types of electronic devices and electric components. However, the thermal resistant temperature of such a bonded magnet is restricted by not only the thermal resistant temperature of the magnetic powder used but also that of the resin binder used to bind the magnetic powder. As for a compressed bonded magnet that uses a thermosetting epoxy resin, for example, the thermosetting epoxy resin has a low heat resistant temperature, and therefore, the maximum allowable temperature, at which the magnet can be used in normal condition, is as low as approximately 100° C. at most. Besides, since a bonded magnet includes an electrically insulating resin binder, it is difficult to carry out a surface treatment such as electrical plating or an evaporation and deposition process of a metal coating.
On top of that, a normal bonded magnet includes a resin binder, and the volume fraction of its magnetic powder cannot be increased to more than 83 vol %. Since the resin binder does not contribute to expressing properties as a magnet, the resultant magnetic properties of a bonded magnet cannot but be lower than those of a sintered magnet.
It should be noted that even in a compressed bonded magnet including a magnetic powder at a relatively high volume fraction, the volume fraction of the magnetic powder is approximately 83 vol % at most and the maximum energy product thereof can be no greater than about 96 kJ/m3 (=12 MGOe).
Recently, very small ringlike magnets with a diameter of 10 mm or less have often been used in small spindle motors, stepper motors and various types of small sensors. In those applications, there is a high demand for permanent magnets with excellent compactibility and improved magnetic properties. That is to say, the magnetic properties of conventional bonded magnets are not enough in those applications more and more often.
A full-dense magnet is known as a magnet including a higher volume fraction of magnetic powder than a bonded magnet. Patent Document No. 1 discloses a full-dense magnet made of a rapidly solidified nanocomposite alloy. Such a full-dense magnet is produced by compressing, and increasing the density of, a magnetic powder of a rapidly solidified alloy without using a resin binder.
Patent Document No. 2 discloses that a nanocomposite magnetic powder is compressed and compacted at a temperature of 550° C. to 720° C. with a pressure of 20 MPa to 80 MPa applied. The density of a full-dense magnet obtained in this manner can be as high as 92% or more of the true density of the magnet.
Patent Document No. 3 discloses a binderless magnet with a magnetic powder purity of 99%, which is coated with a wrapping material. And Patent Document No. 4 discloses a compressed powder magnetic core made of a nanocrystalline magnetic powder.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2004-14906    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2000-348919    Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 10-270236    Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2004-349585
The full-dense magnet disclosed in Patent Document No. 1 includes a magnetic powder at a high volume fraction and is expected to exhibit better magnetic properties than a bonded magnet. However, since the magnet is produced by a hot pressing technology such as a hot-press process, the press cycle is too long to achieve good mass productivity. As a result, the manufacturing cost of the magnets will increase, thus making it difficult to mass-produce such magnets in practice.
The magnet disclosed in Patent Document No. 2 is produced by heating the magnetic powder to a high temperature and compressing it by spark plasma sintering, for example. This process also has too long a press cycle to achieve good mass productivity.
Patent Document No. 3 discloses no specific manufacturing process, and it is not clear how such a high magnetic powder volume fraction is realized. Also, in the compressed powder magnetic core disclosed in Patent Document No. 4, the magnetic powder particles themselves are bound together with glass. The volume fraction of that glass would be approximately equal to that of a resin binder in a conventional bonded magnet.
As can be seen, any of these conventional techniques for compacting a magnetic powder without using a resin binder achieves either just low mass productivity or a magnetic powder volume fraction that is essentially no different from that of a bonded magnet.
Meanwhile, to produce a sintered magnet in which magnetic powder particles have been bound together with substantially no voids left, a sintering process must be performed at as high a temperature as 1,000° C. to 1,200° C. In the sintering process, a liquid phase is formed and a grain boundary phase, including a rare-earth rich phase, is also produced. The grain boundary phase plays an important role to produce coercivity. However, the green compact will shrink significantly during the sintering process. That is to say, since the compact changes its shapes significantly after the press compaction process, the size precision and flexibility in shape of a sintered magnet are much inferior to those of a bonded magnet.